Combi Boiler Heating Radiators But Not Hot Water: Causes, Checks & Fixes
If your combi boiler is heating radiators but not hot water, you are not alone — this is one of the most common boiler complaints the BOYLA Team hear about, especially during the colder months. The good news is that it rarely means a whole new boiler. In most cases, a combi boiler not heating water at the taps while the radiators stay warm points to a single faulty component, and a Gas Safe registered engineer can usually have it sorted within a visit.
This guide explains how a combi boiler is supposed to switch between heating and hot water, the most common reasons it stops doing so, what safe checks you can carry out yourself right now, and what a repair is likely to cost across the UK.
⚠️ Never open the boiler casing, attempt to repair gas components, adjust the gas valve, or work on the internal pipework yourself. These tasks must only be carried out by a Gas Safe registered engineer — it is a legal requirement in the UK, and an unregistered person working on gas appliances risks prosecution, invalidates insurance, and creates serious safety hazards including carbon monoxide poisoning. If you smell gas at any point, turn off the gas at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999. If your carbon monoxide alarm sounds, do the same immediately.
How a Combi Boiler Switches Between Heating and Hot Water
A combi boiler — short for combination boiler — does two jobs in one box: it heats your home through the radiator circuit and provides domestic hot water on demand straight from the mains. There is no separate hot water cylinder.
The component that decides which job to do at any given moment is called the diverter valve. When you open a hot tap or turn on the shower, a signal tells the diverter valve to close off the central heating circuit and route heat to the domestic hot water side instead. When you turn the tap off, the valve switches back. On most combi boilers, domestic hot water is given priority over the heating circuit, which is why your radiators may cool down slightly while you are showering.
When this switching mechanism works correctly, you get seamless hot water within a few seconds of opening a tap. When it fails — most commonly by sticking in the heating position — the boiler fires away quite happily and the radiators stay warm, but the hot water circuit receives little or no heat, leaving your taps cold or barely lukewarm.
Most Likely Cause: A Stuck or Faulty Diverter Valve
A stuck diverter valve is the single most common reason for a combi boiler not heating water while the radiators continue to work normally. It accounts for a large share of boiler call-outs every year across the UK, and in the majority of cases it is fixable without replacing the boiler.
The diverter valve is a mechanical part that moves many times a day — every time a tap is opened or closed — so it wears over time. In a typical UK home, a diverter valve lasts around 8 to 12 years, though this can shorten to 5 to 7 years in hard water areas such as London, the Home Counties, and the East Midlands, where limescale accelerates wear on internal seals and springs.
Common reasons for diverter valve failure include: - Natural wear and tear on springs, seals, and pins - Debris or sludge from the heating system jamming the valve in position - Limescale build-up on the valve seat - Electrical failure of the actuator motor that drives the valve
The tell-tale signs are straightforward: radiators heat up as normal, but taps and showers run cold or stay lukewarm. You may also notice that hot water only appears at the taps when the central heating is switched on — a classic sign the valve is stuck in the heating position and heat is only reaching the hot water circuit by accident rather than by design.
Other Common Reasons Your Combi Boiler Is Not Heating Water
While the diverter valve is the most likely culprit, several other faults can produce the same symptom. Working through these possibilities helps you — and your engineer — narrow things down quickly.
Low boiler pressure is a frequent offender. Most combi boilers need a minimum pressure of around 1 bar to operate correctly, and the domestic hot water function can stop working before the heating circuit notices a problem. Check the pressure gauge on the front of your boiler. If it reads below 1 bar, topping up via the filling loop may restore your hot water immediately.
Airlocks in the pipework can block water from flowing to your taps. You might hear gurgling or bubbling sounds from the boiler or pipes, and radiators may feel warm at the bottom but cold at the top. Bleeding your radiators releases trapped air and can sometimes resolve this.
A faulty thermostat or temperature sensor may prevent the boiler from responding correctly to a demand for hot water. A wireless thermostat that has lost its connection to the boiler after a power cut is a surprisingly common cause — reconnecting or replacing the battery sometimes fixes the issue without any engineer involvement.
Limescale build-up in the plate heat exchanger is particularly relevant in hard water areas. The plate heat exchanger is the component that transfers heat to the domestic hot water side, and when it becomes scaled up, flow is restricted and water cannot reach the correct temperature. You may notice intermittent hot water — fine one moment and cold the next.
Finally, during cold snaps, a frozen condensate pipe can shut the boiler down entirely or cause it to behave erratically, losing hot water first. Condensing combis produce acidic condensate water that drains away through a plastic pipe, often routed externally where it is vulnerable to freezing. Listen for unusual gurgling from the boiler and check whether outside temperatures have dropped below zero.
Safe Checks the Homeowner Can Do Right Now
Before calling an engineer, there are several safe checks you can carry out yourself. None of these involve opening the boiler case, touching gas components, or adjusting anything internally — they are all within the guidance given in most boiler user manuals.
Check your thermostat first. Make sure it is switched on, calling for hot water, and set above the current room temperature. If it is a wireless or smart thermostat, replace the batteries and check it has not lost its connection to the boiler hub. Try increasing the set temperature by a few degrees to see if the boiler responds.
Check the boiler pressure. Look at the pressure gauge — it should sit between 1.0 and 1.5 bar. If it has dropped below 1 bar, use the filling loop (usually a braided hose or lever underneath the boiler) to top it up slowly until it reaches around 1.2 bar, then close the loop. Check your boiler manual for the specific procedure for your model.
Check your programmer or controls. Make sure hot water is not accidentally set to "off" or "holiday" mode. Some programmers have separate schedules for heating and hot water — confirm both are set correctly.
Check your gas supply. If your gas hob or gas fire also fails to light, the problem may be a wider gas supply issue rather than your boiler. Contact your gas supplier.
Try a single reset. Most modern combi boilers have a reset button. If the boiler has locked out and is showing a fault code, one reset is reasonable. However, if the boiler locks out again immediately or the same fault code returns, stop resetting — the boiler is flagging a deeper problem that needs professional diagnosis.
Thaw a frozen condensate pipe (winter only). If you suspect the condensate pipe is frozen, you can carefully pour warm water — not boiling — over the external section of the pipe, or apply a heat pad. Once the ice clears, reset the boiler. Consider lagging the pipe with foam insulation to prevent a repeat.
What a Gas Safe Engineer Will Do
Once a Gas Safe registered engineer arrives, they will carry out a systematic diagnosis that goes well beyond the basic checks you can do yourself. They have specialist tools to test individual components — gas valves, heat exchangers, temperature sensors, and the PCB (printed circuit board) — and can safely open the boiler casing to inspect internally.
For a diverter valve fault, the engineer will typically remove the valve and assess whether it can be cleaned using a chemical cleaner and refitted, or whether it needs replacing entirely. In many cases only the diaphragm or actuator needs replacing, but given the labour involved most engineers recommend fitting a complete new valve assembly to avoid a repeat call-out in the near future.
If limescale or sludge is found to be the underlying problem, the engineer may recommend a power flush of the whole system. This involves circulating a cleaning chemical through your radiators and pipework under pressure to dislodge deposits. It is a worthwhile investment if your system has not been flushed in many years, particularly in hard water regions.
For a faulty PCB, the engineer will test the board electronically before condemning it, as PCBs are expensive to replace. If the board has failed, they will source a compatible replacement — either new or refurbished — for your specific boiler model.
If the plate heat exchanger is scaled or cracked, the repair cost rises significantly. On an older boiler (10 years or more), the engineer should give you an honest comparison between the cost of repair and the cost of a new boiler installation, so you can make an informed decision.
Repair Costs: What to Budget For in the UK
Repair costs vary considerably depending on which component has failed, your boiler brand, how accessible the parts are, and where in the UK you live. The figures below represent typical ranges for 2025/26; treat them as a guide rather than a fixed quote.
Prices vary by region — expect to pay at the top end of these ranges (or 20 to 30% more) in London and the South East, and somewhat less in the North of England and Scotland.
A diagnostic call-out alone typically costs £60–£90 before any repair work begins. This is normal practice — the engineer needs time to test the system before they can give you an accurate repair quote.
Most boiler repairs, once parts and labour are included, fall in the £150–£400 range for straightforward faults such as thermostats, pressure relief valves, or a diverter valve that can be cleaned rather than replaced. More complex jobs — a full diverter valve replacement, a gas valve, or a circuit board — tend to sit between £250 and £500.
If the heat exchanger has failed, costs rise to £400–£650 or more nationally, and higher still in London. At this price point on a boiler that is over 10 years old, it is worth seriously considering a full boiler replacement instead.
A power flush, if needed, typically adds £300–£500 to your bill but can significantly extend the life of your system and improve efficiency — money that often pays back over subsequent winters through lower gas bills.
For emergencies called out of hours, expect to pay a premium. Emergency boiler repair rates commonly run from £225 to £675 depending on the fault and when you call.
Step by step
- Check the thermostat and programmer
Make sure your thermostat is switched on and set above the current temperature. For wireless or smart thermostats, replace the batteries and confirm the connection to the boiler hub is active. Also check your programmer has hot water scheduled — not set to holiday or off mode.
- Check the boiler pressure gauge
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar. If it is below 1 bar, use your filling loop to top it up slowly to around 1.2 bar, then close the filling loop. Never overfill — above 2.5 bar the pressure relief valve will discharge.
- Bleed your radiators
Using a radiator bleed key, release trapped air from each radiator starting with the one furthest from the boiler. Hold a cloth under the bleed valve and open it slowly until water — not air — begins to escape, then close it. Re-check boiler pressure afterwards and top up if needed.
- Reset the boiler once
If your boiler is showing a fault code and has locked out, press the reset button as described in your user manual. Wait for the boiler to go through its start-up sequence. If hot water returns and stays on, you may have had a one-off lockout. If the fault returns, do not keep resetting — call a Gas Safe engineer.
- Thaw a frozen condensate pipe (winter only)
If temperatures are below zero and your boiler has stopped working, check whether the condensate pipe (usually a white or grey plastic pipe exiting the boiler and running outside) is frozen. Pour warm — not boiling — water along the frozen section until clear, then reset the boiler. Lag the pipe with foam insulation to prevent it freezing again.
- Call a Gas Safe registered engineer
If none of the above restores your hot water, the fault is most likely internal — a stuck diverter valve, scaled heat exchanger, faulty PCB, or temperature sensor. These require a professional diagnosis. Always check the engineer is Gas Safe registered at gassaferegister.co.uk before they begin work.
Typical costs
| Diagnostic call-out (typical UK range) | £60–£90 |
| Diverter valve replacement — parts and labour (typical UK range) | £250–£375 |
| Standard repair — most faults, parts and labour included (typical UK range) | £150–£400 |
| PCB (printed circuit board) replacement (typical UK range) | £250–£500 |
| Plate heat exchanger replacement (typical UK range) | £400–£650+ |
| Power flush — whole system (typical UK range) | £300–£500 |
| Expansion vessel or pressure relief valve (typical UK range) | £120–£250 |
| Emergency out-of-hours repair (typical UK range) | £225–£675 |
| Full combi boiler replacement (typical UK range) | £1,800–£4,500 |
Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.
Frequently asked questions
Why is my combi boiler heating radiators but not hot water?
The most common reason is a diverter valve that has stuck in the heating position. The valve normally switches heat between your radiators and your taps — when it jams, the radiators stay warm but the hot water circuit gets little or nothing. Low boiler pressure, a scaled-up plate heat exchanger, or a faulty temperature sensor can also produce the same symptom.
Can I fix a diverter valve on a combi boiler myself?
No. The diverter valve sits inside the boiler casing and involves pressurised water circuits and sometimes electrical components. It must be repaired or replaced by a Gas Safe registered engineer. Attempting it yourself could void your warranty, damage the boiler, or create a safety hazard.
How do I know if my diverter valve is faulty?
The clearest sign is that your radiators heat up normally but your taps and showers stay cold or lukewarm, especially when the central heating is switched off. Another tell-tale sign is that hot water only appears at the taps when the heating is also running — suggesting the valve is stuck in heating mode and hot water is only getting heat by accident.
Will topping up the boiler pressure fix my hot water?
It can do, yes. If your pressure gauge reads below 1 bar, topping it up via the filling loop to around 1.2 bar may restore your hot water immediately. However, if the pressure keeps dropping over the following days or weeks, there is likely a small leak somewhere in the system that needs a Gas Safe engineer to investigate.
How much does it cost to replace a diverter valve in the UK?
Most homeowners pay between £250 and £375 for a diverter valve replacement including parts and labour, on top of an initial call-out charge of around £60–£90. Prices vary by region — expect to pay at the higher end in London and the South East, and less in the North and Scotland. Always ask for a fixed quote before work begins.
Could a frozen condensate pipe cause me to lose hot water?
Yes. During cold spells, the condensate pipe — a plastic pipe that removes acidic water produced by a condensing boiler — can freeze solid, causing the boiler to lock out. You may lose both heating and hot water, or just one of them. You can safely thaw the pipe with warm water and then reset the boiler. If this keeps happening, the pipe should be insulated or rerouted internally by an engineer.